Study On Combustion Characteristics Of Overloaded Wire Insulation In Microgravity

The overload of wire insulation is an important factor that may cause fire for various electric equipment.Especially,in microgravity,the weak natural convection makes the reaction products cannot be efficiently removed.The reduced effect of gravity also makes the time of the insulation layer contacting with the metallic core extremely increase,hindering the fire prevention and detection in space.Studying the combustion characteristics of the overloaded wire insulation under microgravity condition could promote the design of fire protection in space,and also improve the understanding of the overloaded performances of typical insulation materials in microgravity.Therefore,to some extents,this topic has value in both scientific and engineering aspects.In the present thesis,the overload experiments of the wire insulations that operated on board of the wire-characteristic payload of the SJ-10 scientific satellite are analyzed.The combustion characteristics of the wire insulation during the early stage of electrical overload are investigated in microgravity.The modes the smoke emission and the mode of the temperature rise of the overloaded wire insulation are concluded.The low-pressure experiments are conducted in normal gravity.The microgravity results are compared with the low-pressure phenomena to investigate the fire characteristics of electrically overloaded wire in microgravity.In Chapter 2,the principle of laser extinction method is demonstrated,which is applied as the core method of optical observation for the experiments both in normal gravity and in microgravity.A calculating program of the soot concentration and its basic algorithm is introduced.The experimental setup of the low-pressure functional simulation is illustrated.The sample parameters and the setup of the experiment conducted on board of the wire-characteristic payload of the SJ-10 are introduced.In Chapter 3,the smoke emission characteristics of PE(polyethylene)insulation at the initial stage of overload are studied in microgravity.Two smoke emission modes of overloaded wire insulation in microgravity are proposed,the end jetting mode and the bubbling jet mode.The causes of the two smoke-emission mode are analyzed in detail.The results show that the direction of the smoke emission in microgravity is different from those predicted by the ground low-pressure experiment and the dynamics analysis of smoke micro-cluster.In space,the smoke production first occurs on the ends of the inner surface of the insulation,where the oxygen is available and also the insulation is close to the internal heat source.The direction of the smoke emission is dominated by the geometry of the pyrolysis front.The smoke jetting angle evolves as the reaction area expanding.Afterwards,the smoke emission at the ends of insulation turns stable.Meanwhile,the middle insulation gradually emits smoke from its outer surface,producing smoke particles floating with the heated expanding air.Additionally,the unique role of the internal heat source lead to the non-oxygen pyrolysis occurs inside the insulation layer,though the anaerobic reaction has higher activation energy.It creates bubbles at the later stage of overload,accumulating a large amount of pyrolysis product inside.Eventually,bubbles rupture,jetting high-temperature smoke.In Chapter 4,the results of the TG-MS(thermogravimetric mass spectrometry)of three typical insulation materials,the PE,the PVC(polyvinyl chloride)and the PTFE(polytetrafluoroethylene)are analyzed.The degradation and the combustion of the long-term overloaded wire insulation are studied in microgravity.The results show the biggest temperature difference between the aerobic reaction and the anaerobic reaction comes in the PE's tests.The non-oxygen pyrolysis of PE is the chain-breaking reaction of organic compounds,while the combustion of PE produces small-molecular inorganic compounds.Therefore,in microgravity,the overloaded PE insulation with lower melting point quickly melts and shrinks from its both ends,forming two fuel beads and eventually,initiates auto-ignition.The experimental results in microgravity show that,with 4.5 A excess current,the sustainable combustion of the entire 0.4mm-thick PE insulation only occurs after twice flashes.Flashes tend to happen in the gas phase with a certain distance from the surface of the insulation,asynchronously igniting the insulation layer.No matter in air or in inert atmosphere,the PVC sample lose most of its quality at the first stage of temperature rise,generating a large amount of chlorine gas,while remain many residuals after the tests.In microgravity,the PVC insulation strongly emits smoke at the early stage of overload.The smoke zones around the insulation ends become asymmetrical within 15s,creating a smoke jet from the porous surface of insulation.At the end of overload,the bulgy remains of the insulation are still wrapping around the core,which is consistent with the TG-MS results of PVC.The PTFE sample shows the smallest difference of reaction temperatures in air and in inert atmosphere.The TG-MS indicates the temperatures of its main reactions are all above 400?.In microgravity,because of the good flame retardancy and low adhesion of PTFE,the insulation unwrapped from its ends,creating spindle-shape distributions of smoke particles.At later stage,the PTFE insulation ruptures and becomes floating debris.With 4.5A excess current,no obvious chemical reaction occurs in the overload of the wrapping PTFE insulation,showing the best performance in fire safety.In Chapter 5,the temperature characteristic of the overloaded wire insulation in microgravity is investigated.The microgravity mode of the temperature rise of the wire insulation caused by electrical overload is proposed.The factors of the unique temperature rise in microgravity are analyzed.The results show that,in microgravity,the temperature rise of the overloaded wire insulation consists of three stage.In the first stage,the insulation layer acts like a thermal resistance to slow down the temperature rise of the wire.In the second stage,because of the existences of the bubbling smoke jet and the rupture of the insulation,the non-oxygen pyrolysis of the insulation strongly disturbs the temperature field,leading to the "overheat" and the saddle-shaped temperature curves,and also initiates unexpected temperature rises of adjacent wires.In the third stage,the completely degradation of the insulation makes the overall temperature of the wire falls back to the equilibrium temperature of the steady-stage heat transfer in the bare-wire test.The temperature records confirm the existence of the bubbling smoke jet that dominated by non-oxygen pyrolysis.Further,the low-pressure experiments in normal gravity indicate the Functional Simulation is an effective method to simulate the temperature rise of overloaded wire insulation in microgravity.The temperature-rising rate and the equilibrium temperature of the overloaded wire increase with the environmental temperature decreases.The mode of the temperature rise in low pressure shows similarity with the microgravity result.The low-pressure test with the same Gr number as the microgravity condition has the best similarity in the pattern of the temperature curve.